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We present results of developments of low noise millimeter wave receivers for space applications utilizing InP HEMT monolithic microwave integrated circuts (MMICs).
Unique techniques are being utilized to develop self-contained imaging radiometers operating at single and multiple frequencies near 35, 95 and 183 GHz. These techniques include medium to large antennas for high spatial resolution, lowloss open structures for RF confinemnt and calibration, wide bandwidths for good sensitivity plus total automation of the unit operation and data collection. Applications include: detection of severe storms, imaging of motor vehicles, and the remote sensing of changes in material properties.
JPL has had a large amount of experience with spaceborne microwave/millimeter wave radiometers for remote sensing. All of the instruments use filled aperture antenna systems from 5 cm diameter for the microwave Sounder Units (MSU), 16 m for the microwave limb sounder (MLS) to 20 m for the large deployable reflector (LDR). The advantages of filled aperture antenna systems are presented. The requirements of the 10 m Geoplat antenna system, 10 m multified antenna, and the MLS are briefly discussed.
Receiving system for measurement of millimeter and submillimeter wavelength regions with radiometer using Fourier transforms
Studies at 11 locations in the continental United States were directed at an evaluation of rain attenuation effects, scintillations, depolarization, site diversity, coherence bandwidth, and analog and digital communications techniques using the Applications Technology Satellite-6(ATS-6). In addition to direct measurements on the 20- and 30-GHz links, methods of attenuation prediction with radars, rain gages, and radiometers were developed and compared with the directly measured attenuation. Initial data results of the ATS-6 millimeter wave experiment were presented. The first section describes the experiment objectives, flight hardware, and modes of operation. The remaining six sections present papers prepared by the major participating organizations in the experiment. The papers present a comprehensive summary of the significant results of the initial 11 months of ATS-6 experiment measurements and related radiometric, radar, and radio-meteorology studies.
An airborne imaging 92/183 GHz radiometer was recently flown onboard NASA's Convair 990 research aircraft during the February 1983 Bering Sea Marginal Ice Zone Experiment (MIZEX-WEST). The 92 GHz portion of the radiometer was used to gather ice signature data and to generate real-time millimeter wave images of the marginal ice zone. Dry atmospheric conditions in the Arctic resulted in good surface ice signature data for the 183 GHz double sideband (DSB) channel situated + or - 8.75 GHz away from the water vapor absorption line. The radiometer's beam scanner imaged the marginal ice zone over a + or - 45 degrees swath angle about the aircraft nadir position. The aircraft altitude was 30,000 feet (9.20 km) maximum and 3,000 feet (0.92 km) minimum during the various data runs. Calculations of the minimum detectable target (ice) size for the radiometer as a function of aircraft altitude were performed. In addition, the change in the atmospheric attenuation at 92 GHz under varying weather conditions was incorporated into the target size calculations. A radiometric image of surface ice at 92 GHz in the marginal ice zone is included.
Submillimeter-wave cloud ice radiometry is a new and innovative technique for characterizing cirrus ice clouds. Cirrus clouds affect Earth's climate and hydrological cycle by reflecting incoming solar energy, trapping outgoing IR radiation, sublimating into vapor, and influencing atmospheric circulation. Since uncertainties in the global distribution of cloud ice restrict the accuracy of both climate and weather models, successful development of this technique could provide a valuable tool for investigating how clouds affect climate and weather. Cloud ice radiometry could fill an important gap in the observational capabilities of existing and planned Earth-observing systems. Using submillimeter-wave radiometry to retrieve properties of ice clouds can be understood with a simple model. There are a number of submillimeter-wavelength spectral regions where the upper troposphere is transparent. At lower tropospheric altitudes water vapor emits a relatively uniform flux of thermal radiation. When cirrus clouds are present, they scatter a portion of the upwelling flux of submillimeter-wavelength radiation back towards the Earth as shown in the diagram, thus reducing the upward flux o f energy. Hence, the power received by a down-looking radiometer decreases when a cirrus cloud passes through the field of view causing the cirrus cloud to appear radiatively cool against the warm lower atmospheric thermal emissions. The reduction in upwelling thermal flux is a function of both the total cloud ice content and mean crystal size. Radiometric measurements made at multiple widely spaced frequencies permit flux variations caused by changes in crystal size to be distinguished from changes in ice content, and polarized measurements can be used to constrain mean crystal shape. The goal of the cloud ice radiometry program is to further develop and validate this technique of characterizing cirrus. A multi-frequency radiometer is being designed to support airborne science and spacecraft validation missions. This program has already extended the initial millimeter-wave modeling studies to submillimeter-wavelengths and has improved the realism of the cloud scattering models. Additionally a proof-of-concept airborne submillimeter-wave radiometer was constructed and fielded. It measured a radiometric signal from cirrus confirming the basic technical feasibility of this technique. This program is a cooperative effort of the University of Colorado, Colorado State University, Swales Aerospace, and Jet Propulsion Laboratory. Additional information is contained in the original.
The ATS-6 millimeter wave experiment, provided the first direct measurements of 20 and 30 GHz earth-space links from an orbiting satellite. Studies at eleven locations in the continental United States were directed at an evaluation of rain attenuation effects, scintillations, depolarization, site diversity, coherence bandwidth, and analog and digital communications techniques. In addition to direct measurements on the 20 and 30 GHz links, methods of attenuation prediction with radars, rain gages, and radiometers were developed and compared with the directly measured attenuation. Initial data results of the ATS-6 millimeter wave experiment from the major participating organizations are presented.
The experiment concept involves the use of millimeter wave radiation the atmospheric oxygen to provide vertical sensing information to a satellite-borne radiometer. The radiance profile studies require the calculation of ray brightness temperature as a function of tangential altitude and atmosphere model, and the computer program developed for this purpose is discussed. Detailed calculations have been made for a total of 12 atmosphere models, including some showing severe warning conditions. The experiment system analysis investigates the effect of various design choices on system behavior. Calculated temperature profiles are presented for a wide variety of frequencies, bandwidths, and atmosphere models. System performance is determined by the convolution of the brightness temperature and an assumed antenna pattern. A compensation scheme to account for different plateau temperatures is developed and demonstrated. The millimeter wave components developed for the local vertical sensor are discussed, with emphasis on the antenna, low noise mixer, and solid state local oscillator. It was concluded that a viable sensing technique exists, useful over a wide range of altitude with an accuracy generally on the order of 0.01 degree or better.
Receiver techniques and detectors
We have been developing terahertz high resolution spectrometers and radiometers on CubeSat platforms for astrophysics, planetary science, and Earth science applications. We are also developing millimeter-wave communications systems on CubeSat platforms to provide communication links during the entry-descent-landing (EDL) phase of Mars and other planetary missions. In this paper, we present an overview of the state of the millimeter-wave and terahertz instrumentation development for CubeSat platforms. We discuss the design and implementation challenges for these instrument developments. We also look into innovative packaging solutions, antenna technology, and low-power backend options for these instruments. We show that CubeSats and SmallSats will play a key role in future scientific space missions and also they will find increasing applications in commercial communication and other endeavors.
Planar RF circuits are used in a wide range of applications from 1 GHz to 300 GHz, including radar, communications, commercial RF test instruments, and remote sensing radiometers. These circuits, however, provide only fixed tuning elements. This lack of adjustability puts severe demands on circuit design procedures and materials parameters. We have developed a novel tuning element which can be incorporated into the design of a planar circuit in order to allow active, post-fabrication tuning by varying the electrical length of a coplanar strip transmission line. It consists of a series of thin plates which can slide in unison along the transmission line, and the size and spacing of the plates are designed to provide a large reflection of RF power over a useful frequency bandwidth. Tests of this structure at 1 GHz to 3 Ghz showed that it produced a reflection coefficient greater than 0.90 over a 20 percent bandwidth. A 2 GHz circuit incorporating this tuning element was also tested to demonstrate practical tuning ranges. This structure can be fabricated for frequencies as high as 1000 GHz using existing micromachining techniques. Many commercial applications can benefit from this micromechanical RF tuning element, as it will aid in extending microwave integrated circuit technology into the high millimeter wave and submillimeter wave bands by easing constraints on circuit technology.
A three-frequency millimeter-wave feed horn was developed as part of an advanced component technology task that provides components necessary for higher-frequency radiometers to meet the needs of the Surface Water and Ocean Topography (SWOT) mission. The primary objectives of SWOT are to characterize ocean sub-mesoscale processes on 10-km and larger scales in the global oceans, and to measure the global water storage in inland surface water bodies, including rivers, lakes, reservoirs, and wetlands. In this innovation, the feed provides three separate output ports in the 87-to- 97-GHz, 125-to-135-GHz, and 161-to-183- GHz bands; WR10 for the 90-GHz channel, WR8 for the 130-GHz channel, and WR5 for the 170-GHz channel. These ports are in turn connected to individual radiometer channels that will also demonstrate component technology including new PIN-diode switches and noise diodes for internal calibration integrated into each radiometer front end. For this application, a prime focus feed is required with an edge taper of approximately 20 dB at an illumination angle of 40 deg. A single polarization is provided in each band. Preliminary requirements called for a return loss of better than 15 dB, which is achieved across all three bands. Good pattern symmetry is also obtained throughout all three-frequency bands. This three-frequency broadband millimeter-wave feed also minimizes mass and provides a common focal point for all three millimeter-wave bands.
A series of ice detection tests was performed on the shuttle external tank (ET) and on ET target samples using a 35/95 GHz instrumentation radiometer. Ice was formed using liquid nitrogen and water spray inside a test enclosure containing ET spray on foam insulation samples. During cryogenic fueling operations prior to the shuttle orbiter engine firing tests, ice was formed with freon and water over a one meter square section of the ET LOX tank. Data analysis was performed on the ice signatures, collected by the radiometer, using Georgia Tech computing facilities. Data analysis technique developed include: ice signature images of scanned ET target; pixel temperature contour plots; time correlation of target data with ice present versus no ice formation; and ice signature radiometric temperature statistical data, i.e., mean, variance, and standard deviation.
This paper focuses mostly on satellite microwave observations from UARS MLS (Upper Atmosphere Research Satellite Microwave Limb Sounder) at 186.5 and 203.2 GHz.
Sky temperature measurements made at 90-Gc with radio telescope system consisting of 60-inch millimeter searchlight antenna and superheterodyne radiometer
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A microwave integrated circuit active receiver is built and tested at 19-25 GHz. The receiver consists of a planar CPW-fed double folded-slot antenna coupled to a six-stage MESFET (metal semiconductor field effect transistors) amplifier and followed by a planar Schottky-diode detector. The folded-slot antenna on a GaAs half-space results in a wide frequency bandwidth suitable for MMIC amplifiers. The measured system performance show a video responsivity close to 1 GV/W at 20 GHz with a 3-dB bandwidth of 1500 MHz. A novel method which uses the planar video detector after the amplifier stages as an RF (radio frequency) mixer is used to measure the noise-figure of the direct detection radiometer. The system noise figure is 4.8 dB at 22 GHz. The radiometer sensitivity to a hot/cold load is 3.8 mu V/K. The measured antenna patterns show a 90% Gaussicity at 20-22 GHz. The active MIC receiver can be integrated monolithically for low-cost applications and is well suited for millimeter-wave linear imaging arrays.